The ever-increasing demand for portable electronic devices motivates technological improvements in energy conversion and storage units used in these devices. In developing the energy conversion and storage units, such as batteries, lightweight construction, long lifetime, high energy density, high power density and flexibility to meet various design and power needs are important factors to consider. Energy density and power density are two different perspectives of the energy storage devices. Energy density is measured by how long a mobile device can operate, such as making phone calls and uploading data, and how long it can standby. High power density is needed in providing fast bursts of current in power demanding applications on devices such as cameras, hard disk drive, high-resolution displays, etc. Examples of the energy conversion and storage units suitable for portable electronic devices include lithium ion batteries, lithium metal batteries and supercapacitors.
Lithium ion batteries are currently one of the most popular types of solid-state batteries for portable electronic devices, with one of the best energy-to-weight ratios, no memory effect, and a long shelf life. The three primary functional components of a lithium ion battery are anode, cathode and electrolyte, for which a variety of materials may be used. Commercially, the most popular material for the anode is graphite. The cathode may be made with an intercalation lithium compound such as lithium cobalt oxide, lithium iron phosphate, lithium manganese oxide, etc.
Lithium metal batteries, or lithium metal polymer batteries, are rechargeable batteries that evolved from lithium-ion batteries. A lithium-metal battery structure comprises a lithium metal anode, a polymer composite electrolyte and a cathode. Lithium metal batteries can be produced by stacking thin films of these materials together. The resulting device structure is flexible, tough, and durable. The advantages of lithium metal polymer structure over the traditional lithium ion design include lower cost of manufacturing and being more robust to physical damage.
Supercapacitors resemble a regular capacitor except that it offers very high capacitance in a small package. There are two types of supercapacitors, electrochemical double layer capacitor and peuseodocapacitor. In electric double layer capacitor (EDLC), energy storage is by means of static charge rather than an electro-chemical process that is inherent to the batteries. Applying a voltage differential on the positive and negative plates charges the supercapacitor. Whereas a regular capacitor consists of conductive foils and a dry separator, the supercapacitor crosses into battery technology by using electrodes and electrolyte that are similar to lithium ion or lithium metal batteries. Hence, a combination of battery and supercapacitor in a unit is of great interest for achieving high energy density and power density.
Energy storage units with combinations of batteries and capacitors have been proposed, and applied in electronic devices, but no combined battery-supercapacitor unit has been developed to fill the area 1 in the Ragone plot (power density vs. energy density) as shown in FIG. 1. A device falling in this area is very desirable from application point of view, because it offers a very high energy density and a very high power density at the same time. Ideally, a combined battery/supercapacitor should have the power of a supercapacitor with the storage capacity of a battery. Like a capacitor, it can be rapidly charged then discharged to deliver power. Like a battery, it can store and deliver that charge over long periods of time.
Recently, nanostructured materials are being used in rechargeable batteries as cathode or anode in order to enhance the battery capacity and durability. Nanostructured carbon, such as carbon nanotubes (CNTs), carbon nanowires (CNWs), carbon nanohorns (CNHs) and carbon nano-onions (CNOs) are being contemplated for replacing graphite. CNT is a highly crystallized tubular structure of carbon. One single wall nanotube (SWNT) is about a few nanometers in diameter and up to a hundred microns long, multiwall nanotubes (MWNT), which are typically the case in vertical growth, are larger in diameter and equally long or longer. Millions of carbon nanotubes together may form a cluster of macroscopic material that is practically useful. CNTs may be grown from a smooth substrate to form a layer of densely packed, vertically aligned CNT pile (morphologically similar to a pile of fiber on a carpet).
CNH and CNO are highly crystallized nanoparticle structures of carbon. Single wall carbon nanohorns (SWCNHs) are structured from cone-shaped hollow carbon (graphene) crystallite about two to three nanometers long. They aggregate to form dahlia-, or bud-like nanoparticle structures 50 to 100 nanometers in diameter and are spherical or near spherical in shape, with nanocones on the surface (see FIG. 2). CNOs are ball-shaped crystallite (fullerenes) with one carbon ball enclosing another carbon ball.
The advantage of using CNHs and CNOs in energy conversion and storage devices lies not only in the extremely large surface area but also easy permeation for gas and liquid, because of surface defects, openings and windows in the crystal surface structures naturally generated by a submerged discharge process. CNHs are also especially applicable to surface adsorption processes because of the non-uniform diameter throughout the cone structure, resulting from the hexagon-pentagon distribution on the surface, instead of the even hexagon structure of the pure graphene layers like on graphite, or pure CNTs. CNH cones are built in certain cone angles with fullerene hemisphere tip at the end. The angles are defined in pure cone structures in 19, 39, 60, 84 and 113 degrees—the wider the cone angle, the shorter the nanocones. CNHs can be produced by a laser ablation process or a submerged arc-discharge process. The latter process is more promising for volume production with greatly reduced cost.
In this disclosure, we describe a complex carbon nanostructure which includes a layer of CNH (or CNO) particles on a layer of highly packed and vertically aligned CNT structure grown on a flexible metal substrate. The complex CNT/CNH(CNO) nanostructure thus resulted can be directly used for electrodes in rechargeable batteries and supercapacitors. The process is suitable for mass productions of the nanostructured carbon material and mass production of the above-described energy conversion and storage units comprising the nanostructured carbon material.